Despite considerable research, the role of histamine in embryo development and implantation is not clearly understood. However, recent findings seem to be encouraging in answering many of the previously unanswered questions. Our current research indicates that rabbit blastocyst and uterine endometrial cell membranes carry H2 and H1 type histamine receptors (Nature 278:648-649, 1979). We have also found that rabbit preimplantation blastocysts, but not uterine endometrial tissues, have considerable histamine forming capacity as determined by measuring their histidine decarboxylase activity. The intrauterine injection of a specific inhibitor (DL-gamma-methylhistidine) of histidine decarboxylase interrupts implantation in the rabbits; this inhibition is overcome when a combination of the inhibitor and the substrate (L-histidine) for the enzyme is injected concomitantly (Biol. Reprod. In Press).